Glasses having compositions approximating the stoichiometry of lithium disilicate (Li.sub.2 O.multidot.2SiO.sub.2) require very high temperatures for melting and are notoriously unstable, i.e., they are very prone to devitrification, that latter characteristic to date having precluded the manufacture of glass-ceramic articles on a commercial scale therefrom due to the inadequacy of microstructural tailoring required for necessary property control. Thus, the fabrication of glass-ceramic articles containing lithium disilicate crystals as the predominant crystal phase (frequently utilizing P.sub.2 O.sub.5 as a nucleating agent) has been reported in the art, such articles purportedly exhibiting high mechanical Because or rne inherent instability of the glass phase, however, the reproducibility in properties demanded in commercial production has been lacking. Nevertheless, on account of the high mechanical strengths demonstrated by glass-ceramic articles containing lithium disilicate crystals when those crystals are relatively uniformly fine-grained and homogeneously dispersed throughout the articles, research has continued to devise precursor glass compositions and heat treatment schedules therefor which would yield sound glass-ceramic articles wherein the advantageous physical properties of lithium disilicate-containing glass-ceramics would be reproducible.
Accordingly, the primary objective of the present invention was to devise glass compositions which would inherently be stable, thereby permitting a high degree of microstructural control during the development of crystals therein and imparting great flexibility in forming techniques, for example, drawing, floating, pressing, rolling, spinning, etc.
Another objective of the instant invention was to design glass compositions, which, upon being crystallized in situ via a heat treatment being applied thereto, would form glass-ceramic articles wherein the properties thereof are reproducible.
Yet another objective of the subject invention was to produce sound glass-ceramic articles which, during the crystallization heat treatment, develop a self-glazed surface of high gloss.